The ultimate goal of our research is to develop an understanding of the response of cancer to therapy using measurements of DNA synthetic pathways as imaged by positron emission tomography (PET). We want to make imaging cellular proliferation as widely available and generally useful as FDG imaging has been for cellular energetics. To make valid measurements of cellular proliferation using PET, we are developing labeled nucleosides and producing detailed biochemical and kinetic models of their metabolism. Our studies have helped develop the techniques needed to obtain PET images with [C-11]thymidine, which is incorporated into DNA and can be used to infer cellular proliferation. While thymidine is proving clinically useful, it is not an ideal tracer because of its rapid degradation. To improve imaging proliferation, we propose a new approach focusing on thymidine kinase-1 (TK1) and using thymidine analogs that are stable to degradation in vivo. TK1 is the initial enzyme in the incorporation of exogenous thymidine. It is tightly regulated and elevated 10-15 fold in cells undertaking DNA synthesis. TK1"s nucleoside substrates are converted to impermeable intracellular nucleotide phosphates and held within the DNA pathway. We have examined a number of thymidine analogs and have determined that the best candidates are AZT and FLT, drugs previously used in humans. Retention of labeled thymidine analogs, mediated by TK1, should be analogous to measuring hexokinase activity with FDG. Our preliminary results indicate that both compounds and the study of their uptake and retention in tissue culture and animal tumor systems. This will include quantitative imaging and modeling in tumor bearing dogs. As an alternative method of imaging proliferating cells we will continue to study FFUdR, which we have found to be retained in tumors. Recent studies in cell culture indicate that FFUdR may also be useful in measuring the success of gene therapy. We will further study this and related compounds, BVFRU and BVFddU, to determine their ability to evaluate gene therapy. Selective retention of these tracers in cells is based on the altered substrate specificity of herpes simplex virus thymidine kinase (HSVtk), the viral isozyme of TK1, which is used in gene therapy. The long range goal of this portion of the project is to develop an imaging agent for visualizing durable transection of tumors for "suicide-marker" gene-therapy.